The invention relates to a carburetor for mixing fuel with an air flow to form a combustible air/fuel mixture for an internal combustion engine.
A carburetor mixes fuel with an air flow to form a combustible air/fuel mixture. Carburetors were once widely used to supply fuel to motor vehicle engines. Carburetors have been replaced by fuel if injection systems in today""s mass-produced motor vehicles but are still used in custom vehicle applications. These custom carburetor applications include automobile racing, where high power output and quick throttle response are desired.
A carburetor has a body having a through bore defining a venturi. Air flows through the bore before entering the engine. A fuel line extends between a fuel reservoir and the venturi to flow fuel from the fuel reservoir and mix the fuel with the air to form a combustible air/fuel mixture.
A throttle plate in the bore downstream from the venturi opens and closes the bore to regulate the flow of the fuel/air mixture into the engine. As the throttle opens, air flow through the venturi increases. More fuel is mixed with the increasing air flow to maintain a combustible air/fuel mixture as engine speed increases. Conversely, when the throttle plate closes the air flow decreases and the amount of fuel mixed with the air decreases as engine speed decreases.
The venturi includes a reduced diameter throat. The speed of the air flow increases through the throat and its air pressure decreases by a physical effect known as the venturi effect. The reduced air pressure generates a partial vacuum or suction in the venturi throat. The fuel line opens in the venturi throat so that the suction draws fuel from the fuel reservoir through the fuel line and into the venturi to form the air/fuel mixture.
The amount of fuel mixed with the air flow is metered to form the optimum air/fuel mixture required for combustion. If too much fuel is added to the air flow, the air/fuel mixture is too rich. If not enough fuel is added, the air/fuel mixture is too lean. In either case engine performance will suffer and engine power is reduced. The optimum air/fuel mixture delivered by the carburetor should be maintained over the entire range of engine operation for best engine performance.
Carburetors are broadly classified by how the fuel mixture is metered by the carburetor. A variable venturi carburetor includes a variable venturi inlet in which the area of the inlet varies with throttle position. The inlet area varies to maintain a substantially constant suction in the venturi throat for all throttle positions. As the throttle plate opens and closes, a valve opens and closes the fuel line to control the flow of fuel in the fuel line and maintain the air/fuel mixture within the optimum range. Variable venturi carburetors are complex and can have a relatively slow throttle response.
A fixed venturi carburetor includes a fixed venturi inlet in which the inlet area remains constant and does not vary with throttle plate position. The average speed of the air flow through the venturi and the suction generated in the venturi throat varies as the throttle plate opens and closes. The air flow and suction increases as the throttle plate opens, and the increasing suction draws more fuel through the fuel line. The fuel line has an orifice or jet which meters fuel flow and enables a combustible air/fuel mixture to be maintained as the throttle position varies.
The fixed venturi carburetor is less complex than a variable venturi carburetor. However, it is difficult to size the jet in a fixed venturi carburetor to maintain the optimum air/fuel mixture over the entire range of engine operation. The jet size is a compromise between low engine speed performance and high engine speed performance.
Thus, there is a need for an improved carburetor for custom vehicle applications. The improved carburetor should not require complex metering systems and yet should maintain the optimum air/fuel mixture over the entire range of engine operation.
The present invention is directed to an improved carburetor for internal combustion engines. The improved carburetor does not require a variable metering system and yet maintains an optimum air/fuel mixture over essentially the entire range of engine operation.
A carburetor having features of the present invention includes a body having a wall bounding a through bore extending along an axis. The wall defines a venturi having an inlet, an outlet spaced axially from the inlet, and a reduced diameter throat between the inlet and outlet. The throat has an outer portion adjacent the body wall and an inner portion surrounded by the outer portion. A throttle plate is in the bore downstream from the venturi outlet and is adapted to control air flow discharged from the venturi.
The carburetor includes a fuel reservoir and a first fuel line connects the fuel reservoir with a first nozzle for discharging fuel into the venturi. A second fuel line connects the fuel reservoir with a second nozzle for discharging fuel into the venturi. The first nozzle opens into the outer portion of the venturi throat flush with or close to the body wall. The second nozzle opens into the inner portion of the venturi throat.
The first nozzle supplies fuel during low speed engine operation. As the throttle plate opens from its closed, idle position, the throttle plate is initially partially open and obstructs the inner portion of the venturi outlet. Suction generated by the venturi effect by air flowing through the outer venturi throat portion adjacent the body wall flows fuel through the first nozzle and into the air flow. During low engine operation, the second nozzle is essentially inactive.
The second nozzle supplies fuel during mid-speed to high speed engine operation. As the throttle plate opens to full throttle, air flow through the inner portion of the venturi outlet becomes essentially unobstructed. Suction generated-by the venturi effect by air flowing through the inner venturi throat portion flows fuel through the second nozzle and into the air flow. During high speed engine operation some fuel is drawn through the first nozzle but the second nozzle supplies essentially all the fuel flow.
The fuel lines flowing fuel from the fuel reservoir to the first and second fuel nozzles can each include a fixed orifice or jet to meter fluid flow. It is not necessary to vary the orifice size with throttle position, and so the complexity of the carburetor is reduced. The two nozzles discharge into radially separated portions of the air flow to maintain the optimum air/fuel ratio throughout the entire range of engine operation. The engine generates more power at all engine speeds with less complexity and improved throttle response as compared to a conventional fixed or variable venturi carburetor.
In possible embodiments of the present invention, the carburetor includes a booster venturi in the venturi throat. The outer wall of the booster venturi and the wall of the venturi throat can define a narrow annular gap between them that acts as a constriction to enhance the venturi effect at low engine speeds. The first nozzle discharges on the inner wall of the booster venturi for improved low speed response. Preferably the booster venturi is removably mounted in the venturi throat to enable the maximum air flow through the carburetor to be regulated by varying the inner diameter of the booster venturi. In other embodiments the booster venturi can have its outer wall away from the wall of the venturi and the second nozzle discharges into the gap between the venturi wall and the booster venturi.
In yet other possible embodiments of the present invention, at least one of the first and second nozzles includes one or more planar surfaces facing the venturi inlet and inclined to the direction of air flow to deflect the air flow past the nozzle. The planar surfaces enhance the venturi effect generated at the nozzle outlet. In one possible embodiment the first nozzle has a polygon outer cross section with the polygon sides incorporating the planar surfaces.
Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings illustrating the invention, of which there are two sheets of three embodiments.